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In this video, we will delve into the core of genetics as we break down the fundamentals of chromosomes, explaining what they are and how they impact our tra...
- 6 min
- 35.7K
- Joao's Lab
Chromosomes are the structures that contain your genetic information. In this video, we show you when chromosomes appear, what they look like, and how they p...
- 2 min
- The Genome
Welcome to our one-minute video all about chromosomes! In this video, we'll explore what chromosomes are, where they're located inside the cell, and how they...
The chromosomes unique structure has a few key parts. Each chromosome has two short arms called p arms and two long arms called q arms. These arms are held together at the center by the centromere. The tips of the chromosome are capped by sections of DNA called telomeres. Telomeres protect chromosomes during DNA replication.
- Overview
- Introduction
- DNA and genomes
- Chromatin
- Chromosomes
- Chromosomes and cell division
DNA, chromosomes, and genomes. Homologous chromosomes, sister chromatids, and haploid/diploid.
When a cell divides, one of its main jobs is to make sure that each of the two new cells gets a full, perfect copy of genetic material. Mistakes during copying, or unequal division of the genetic material between cells, can lead to cells that are unhealthy or dysfunctional (and may lead to diseases such as cancer).
But what exactly is this genetic material, and how does it behave over the course of a cell division?
DNA (deoxyribonucleic acid) is the genetic material of living organisms. In humans, DNA is found in almost all the cells of the body and provides the instructions they need to grow, function, and respond to their environment.
When a cell in the body divides, it will pass on a copy of its DNA to each of its daughter cells. DNA is also passed on at the level of organisms, with the DNA in sperm and egg cells combining to form a new organism that has genetic material from both its parents.
Physically speaking, DNA is a long string of paired chemical units (nucleotides) that come in four different types, abbreviated A, T, C, and G, and it carries information organized into units called genes. Genes typically provide instructions for making proteins, which give cells and organisms their functional characteristics.
In eukaryotes such as plants and animals, the majority of DNA is found in the nucleus and is called nuclear DNA. Mitochondria, organelles that harvest energy for the cell, contain their own mitochondrial DNA, and chloroplasts, organelles that carry out photosynthesis in plant cells, also have chloroplast DNA. The amounts of DNA found in mitochondria and chloroplasts are much smaller than the amount found in the nucleus. In bacteria, most of the DNA is found in a central region of the cell called the nucleoid, which functions similarly to a nucleus but is not surrounded by a membrane.
In a cell, DNA does not usually exist by itself, but instead associates with specialized proteins that organize it and give it structure. In eukaryotes, these proteins include the histones, a group of basic (positively charged) proteins that form “bobbins” around which negatively charged DNA can wrap. In addition to organizing DNA and making it more compact, histones play an important role in determining which genes are active. The complex of DNA plus histones and other structural proteins is called chromatin.
For most of the life of the cell, chromatin is decondensed, meaning that it exists in long, thin strings that look like squiggles under the microscope. In this state, the DNA can be accessed relatively easily by cellular machinery (such as proteins that read and copy DNA), which is important in allowing the cell to grow and function.
Each species has its own characteristic number of chromosomes. Humans, for instance, have 46 chromosomes in a typical body cell (somatic cell), while dogs have 781 . Like many species of animals and plants, humans are diploid (2n), meaning that most of their chromosomes come in matched sets known as homologous pairs. The 46 chromosomes of a human cell are organized into 23 pairs, and the two members of each pair are said to be homologues of one another (with the slight exception of the X and Y chromosomes; see below).
Human sperm and eggs, which have only one homologous chromosome from each pair, are said to be haploid (1n). When a sperm and egg fuse, their genetic material combines to form one complete, diploid set of chromosomes. So, for each homologous pair of chromosomes in your genome, one of the homologues comes from your mom and the other from your dad.
The two chromosomes in a homologous pair are very similar to one another and have the same size and shape. Most importantly, they carry the same type of genetic information: that is, they have the same genes in the same locations. However, they don't necessarily have the same versions of genes. That's because you may have inherited two different gene versions from your mom and your dad.
As a real example, let's consider a gene on chromosome 9 that determines blood type (A, B, AB, or O)2 . It's possible for a person to have two identical copies of this gene, one on each homologous chromosome—for example, you may have a double dose of the gene version for type A. On the other hand, you may have two different gene versions on your two homologous chromosomes, such as one for type A and one for type B (giving AB blood).
As a cell prepares to divide, it must make a copy of each of its chromosomes. The two copies of a chromosome are called sister chromatids. The sister chromatids are identical to one another and are attached to each other by proteins called cohesins. The attachment between sister chromatids is tightest at the centromere, a region of DNA that is important for their separation during later stages of cell division.
As long as the sister chromatids are connected at the centromere, they are still considered to be one chromosome. However, as soon as they are pulled apart during cell division, each is considered a separate chromosome.
What happens to a chromosome as a cell prepares to divide.
Why do cells put their chromosomes through this process of replication, condensation, and separation? The short answer is: to make sure that, during cell division, each new cell gets exactly one copy of each chromosome.
For a more satisfying answer, check out the articles and videos on the cell cycle and mitosis. There, you can see how the behavior of chromosomes helps cells pass on a perfect set of DNA to each daughter cell during division.
[Attribution and references]
A Chromosome is a single complete strand of DNA, along with an associated group of packaging proteins. Humans have 46 chromosomes, 23 from mom, 23 from dad. Together these 46 chromosomes contain all the genes needed to make you who you are. I’m Jon Perry, and that’s Chromosomes, Stated clearly. Read Full Transcript.
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explorebiology.org/bio-dictionary What is a chromosome? DNA is organized as discrete units called chromosomes. This video examines how the chromosome organization…
